The present invention relates to an optical fiber coupler which performs optical coupling between a plurality of cores through cladding at an optical coupling portion and relates more particularly to an optical fiber coupler of a wavelength division multiplex/demultiplex type employing such claddings. The invention also pertains a method for the manufacture of such an optical fiber coupler.
FIG. 1 shows a conventional optical fiber coupler. An optical coupling portion 11 includes two cores 13a and 13b in a common cladding 12 as depicted in FIG. 2 which is a cross-sectional view taken on the line 2--2 in FIG. 1. Reference numerals 15 and 16 denote optical fibers having cores continuous with both ends of the core 13a, and 17 and 18 denote optical fibers similarly having cores continuous with both ends of the core 13b. Usually, one optical fiber forming the optical fibers 15 and 16 and another optical fiber forming the optical fibers 17 and 18 have their intermediate portions fused together by heating, and the fused portion is elongated or stretched to form the optical coupling portion 11. In this way the optical fibers 15, 16 and 17, 18 are optically coupled together at the optical coupling portion 11.
A refractive index n along a straight line Ox passing through the centers of the cores 13a and 13b and perpendicular to the optical coupling portion 11 is high in the cores 13a, 13b as indicated by N.sub.CR in FIG. 3 and low and constant in the cladding 12 as indicated by N.sub.CL. The characteristics of the optical fiber coupler are dependent upon parameters such as the difference between the refractive indexes of the cores 13a, 13b and the cladding 12, the diameters of the cores 13a and 13b, their spacing and the length of the optical coupling portion 11.
The conventional optical fiber coupler encounters a difficulty in reducing the length of the optical coupling portion 11. The branching ratio CR in the optical fiber coupler is expressed by the following equations: ##EQU1## where c(z) is a coupling coefficient in the z coordinates, z is a coordinate in the longitudinal direction of the optical coupling portion 11 and L is the length of the optical coupling portion 11.
It is well-known in the art that the coupling coefficient C needs to be large in the case of fabricating a coupler which is used as a so-called WDM (wavelength division multiplex/demultiplex) coupler in particular, a coupler whose branching ratio is narrow in the wavelength spacing of adjacent peaks, i.e. a highly wavelength dependent coupler. To make the coupling coefficient C large, it is necessary to maximize the coupling coefficient c(z). This can be accomplished by decreasing the core diameter in the optical coupling portion to facilitate the leakage of light from each core and by reducing the core spacing in that portion to facilitate the coupling of light. With too small a core diameter in the optical coupling portion as compared with the core diameter at the end of each optical fiber, however, the coupling coefficient c(z) becomes large but the discontinuity in the core diameter increases, causing an increase in an excess loss of the optical fiber coupler as a whole. Accordingly, it is customary in the prior art to increase the length L of the optical coupling portion 11 to make the coupling coefficient C large without abruptly decreasing the core diameter. Hence, the optical coupling portion 11 is long in the prior art optical fiber coupler.
Thus, no WDM coupler with a short optical coupling portion could have been implemented with the optical fiber coupler.
Besides, the conventional optical fiber coupler has relatively high polarization dependence. FIG. 4 shows the branching ratio vs. wavelength characteristic of the conventional optical fiber coupler measured using random light. As is evident from FIG. 4, the branching ratio is not 0% but is 7 to 8% at a wavelength of about 1534 nm, and not 100% but 90% or so at a wavelength of about 1470 nm. This is considered to be caused by the polarization dependence of the optical fiber coupler. The dependence of the branching ratio on polarization was .+-.9.3% at a wavelength of 1548 nm, which is relatively large.
To realize an optical fiber coupler of low wavelength dependence, it is necessary to minimize the coupling coefficient C in contrast to the case of increasing the wavelength dependence. This can be accomplished by maximizing the core diameter and the core spacing, but they cannot be made larger than those of an optical fiber which is used to form the optical fiber coupler. Accordingly, there is a limit to the reduction of the wavelength dependence of the optical fiber coupler. In a conventional optical fiber coupler which has a branching ratio of 50% at a wavelength of 1.3 .mu.m, it is difficult to make its wavelength dependence 0.2% or below.
To obtain an optical fiber coupler of a wide wavelength band, it has also been proposed to employ a method wherein an optical fiber is fused and stretched and another optical fiber is fused and connected thereto and then stretched so that the core diameter of one optical fiber is smaller than the core diameter of the other optical fiber. With this method, however, it is difficult to manufacture optical fiber couplers of desired characteristics with a satisfactory yield of product.